People with color-deficient vision for red and green may still see yellow. This is most easily explained by

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The opponent-process theory explains that our color vision is based on three opposing channels: red-green, blue-yellow, and black-white. People with color-deficient vision for red and green will have an impaired red-green channel, but their blue-yellow channel will still function normally. Therefore, they will still be able to see yellow, which is processed through the blue-yellow channel.

People with Color-Deficient Vision for Red and Green May Still